This invention relates to a control circuit for a press in which a rotary cam switch for stopping the press at the top dead center is also used for preventing the re-start thereof.
In general, a press is provided with a one-stroke one-stop mechanism, so that, even if the operating push button is maintained depressed, the press is stopped at the top dead center after every stroke.
In order to stop a press at the top dead center and to prevent the restart of the press, a conventional control circuit is provided with two separate switches, namely, a top dead center stop rotary cam switch and a restart prevention rotary cam switch,
One example of the conventional control circuit is as shown in FIG. 1, in which the above-described rotary cam switches are employed in combination.
The operation of the conventional control circuit will be described. FIG. 2 shows the intervals (or slide stroke angles) during which the restart prevention rotary cam switches RCLS1 and RCLS2 and the top dead center stop rotary cam switches RCLS3 and RCLS4 are maintained closed (ON), respectively. In FIG. 1, reference numeral 1 designates a relay coil having contacts 1a and 1b, and reference numeral 2 designates a relay coil having contacts 2a, 2b and 2c.
(1) Before the starting operation is effected (with the slide stroke angle 0.degree.)
Since push buttons PB1 and PB2 for effecting the starting operation are not operated yet, i.e., their contacts a are closed (on) while their contacts b are open (off), the operating relay coil 2 is not energized yet. Accordingly, the normally closed contact 2a is closed (on), the restart prevention relay coil 1 is energized to close its normally open contact 1a. In this case, as is clear from FIG. 2, the restart prevention rotary switches RCLS1 and RCLS2 are closed, and therefore the restart prevention relay coil is self-held.
(2) Effecting the starting operation
Upon operation of the operating push buttons PB1 and PB2, the operating relay coil 2 is energized to close its contact 2c because, in this case, the normally open contact 1b of the relay coil 1 is maintained closed. As a result, a clutch brake solenoid SOL for driving the press is energized to start the press.
(3) Before the slide stroke angle 140.degree.
Before the slide stroke angle reaches 140.degree., the top dead center stop rotary cam switches RCLS3 and RCLS4 are maintained open, and therefore the relay coil 2 is not self-held by its relay contact means 2b. Accordingly, the press can be stopped by restoring the operating push buttons PB1 and PB2 when necessary.
(4) From the slide stroke angle 140.degree. to 260.degree.
At the slide stroke angle 140.degree., the top dead center stop rotary cam switches RCLS3 and RCLS4 are turned on and the relay coil 2 is self-held by its contact 2b. The press thereby continues its operation regardless of depression or non-depression of the operating push buttons PB1 and PB2.
(5) From the slide stroke angle 260.degree. to 330.degree.
When the slide stroke angle has reached 260.degree., the restart prevention rotary cam switches RCLS1 and RCLS2 are opened to deenergize the restart prevention relay coil 1. Even if the restart prevention rotary cam switches RCLS1 and RCLS2 are closed again when the slide stroke angle has reached 280.degree., the operating relay coil 1 is not energized again because the self-holding of the operating relay coil 1 by the contact 1a has been released. That is, the contact 1b is placed in "off" state. Accordingly, even if the operating push buttons PB1 and PB2 are kept depressed, the press can be stopped (i.e., the restart prevention function can be effected) when the rotary cam switches RCLS3 and RCLS4 are opened.
(6) To the top dead center
When the slide stroke angle has reached 330.degree., the top dead rotary cam switches RCLS3 and RCLS4 are opened, and the self-holding of the operating relay coil 2 is released. As a result, the clutch brake solenoid SOL is deenergized, and the press, being run by its inertia, is stopped at the top dead center (i.e., the top dead center stop function is effected).
As is clear from the above description, in the conventional control circuit, the functions of preventing the restart of the press and stopping the press at the top dead center are performed by using the separate rotary cam switches.
A rotary cam switch which requires precision machining is expensive and, besides, adjustment of mounting position thereof is difficult.